Adding a court

Is there anyway I can add a court to the 5 on 5 courts in play now without getting rid of one of the courts already there?

Yeah, but you'd need to create an entirely new .viv file for it. It's easier to replace one of the All-Star, Euro/Int All-Stars, etc. courts.

so if I created a new .viv file and put it in the stadiums folder it would show up or would I have to do something else also

You need to add it to a .dbf to, but I don't know which one.

is the only way to create a custom team?

No, I don't think you need a custom team to have a new court.